Optical glass and process for the production of optical products

ABSTRACT

Disclosed is an optical glass having high-refractivity and low-dispersion optical properties and having a low glass transition point so that a heat-treating furnace can be operated for a long period of time.  
     The present invention provides an optical glass having a refractive index nd of at least 1.875, an Abbe&#39;s number νd of at least 39.5 and a glass transition point Tg of 700° C. or lower, an optical glass which is a borosilicate glass comprising at least one selected from La 2 O 3 , Gd 2 O 3 , Y 2 O 3  or Yb 2 O 3  and at least one selected from ZrO 2 , Ta 2 O 5  or Nb 2 O 5 , wherein the ratio (weight ratio) of the total content of La 2 O 3 , Gd 2 O 3 , Y 2 O 3  and Yb 2 O 3  to the total content of SiO 2  and B 2 O 3  is from 3.2 to 5 and the ratio (weight ratio) of the total content of ZrO 2 , Ta 2 O 5  and Nb 2 O 5  to the total content of SiO 2  and B 2 O 3  is from 1.1 to 1.5, and which has a refractive index nd of at least 1.875 and an Abbe&#39;s number νd of at least 39.5, and the like.

TECHNICAL FIELD

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical glass, a glass preform made of it, an optical product and a process for the production of the optical product. More specifically, it relates to an optical glass which has high-refractivity and low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time, a glass preform which is made of the above optical glass and suitable for the formation of various optical products, an optical product made of the above optical glass, and a process for the production of the above optical product.

[0003] 2. Explanation of Related Art

[0004] Conventionally, an optical glass having high-refractivity and low-dispersion optical properties contains a large amount of La₂O₃, Gd₂O₃, Y₂O₃, Ta₂O₅, ZrO₂, etc., for attaining a high refractive index and a low dispersion property as is shown, for example, in JP-A-54-90218 and JP-B-54-6042, and glass-network-forming components such as B₂O₃ and SiO₂ are contained in a small amount, so that the above optical glass is extremely highly liable to undergo crystallization. Since a glass that can be stably produced is limited in compositional range, commercially available optical glasses having a high refractive index and a low dispersion have a glass transition point Tg of over 720° C. Table 1 shows properties of high-refractivity low-dispersion optical glasses shown in brochures supplied by optical glass manufacturers. TABLE 1 Refractive Abbe's Glass transi- index number tion point [nd] [vd] [Tg] (° C.) A 1.88300 40.8 730 B 1.88067 41.01 758 C 1.88300 40.8 738

[0005] Conventional high-refractivity low-dispersion glasses have a very high temperature for viscous flows as is typically shown by glass transition points Tg, and for example, annealing treatment thereof has required a temperature of 710° C. or higher. Generally, most of furnaces for glass annealing are made of a stainless steel plate, and this material has a deformation temperature around 700° C. When the annealing is carried out at a temperature over 710° C., therefore, there is caused a problem that the above stainless steel plate undergoes deformation, so that it is difficult to operate the furnace for a long period of time.

[0006] Further, the production of a lens material by re-heat pressing also requires a very high temperature, which causes a heat-treating furnace to deteriorate sooner and hinders stable production.

[0007] Meanwhile, when a glass has a glass transition point Tg of 700° C. or lower, stable production has been achieved without causing any special load on the operation of facilities.

SUMMARY OF THE INVENTION

[0008] Under the circumstances, it is a first object of the present invention to provide an optical glass that has high-refractivity low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time.

[0009] It is a second object of the present invention to provide a glass preform that is made of the above optical glass and is suitable for forming various optical products, and an optical product made of the above optical glass.

[0010] It is a third object of the present invention to provide a process for efficiently producing an optical product made of the above optical glass.

[0011] For developing optical glasses having the above desirable properties, the present inventors have made diligent studies with regard to effects of compositions of components constituting glasses on optical properties, thermal properties and devitrification resistance. As a result, it has been found that an optical glass having high-refractivity and low-dispersion optical properties and having a glass transition point of 700° C. or lower can be obtained by controlling the ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃, and the ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅, to the total content of SiO₂ and B₂O₃ such that the above ratios come into specific ranges, or by forming a glass having a specific glass composition, and that the first object of the present invention can be accordingly achieved.

[0012] Further, it has been also found that the above second object of the present invention can be achieved by a glass preform and an optical product made of the above optical glass each.

[0013] It has been further found that a optical product made of the above optical glass can be efficiently produced by employing a specific step, and that the above third object of the present invention can be accordingly achieved.

[0014] The present invention has been completed on the basis of the above findings. That is, the present invention provides:

[0015] (1) an optical glass having a refractive index nd of at least 1.875, an Abbe's number νd of at least 39.5 and a glass transition point Tg of 700° C. or lower (to be referred to as “optical glass I of the present invention” hereinafter),

[0016] (2) an optical glass which is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃ and at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅, wherein the ratio (weight ratio) of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 3.2 to 5 and the ratio (weight ratio) of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 1.1 to 1.5, and which has a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5 (to be referred to as “optical glass II of the present invention” hereinafter),

[0017] (3) an optical glass which is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃, at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅ and ZnO, wherein the ratio (weight ratio) of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 2 to 5, the ratio (weight ratio) of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 0.5 to 3 and the ratio (weight ratio) of ZnO to the total content of SiO₂ and B₂O₃ is at least 0.14, and which has a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5 (to be referred to as “optical glass III of the present invention” hereinafter),

[0018] (4) an optical glass having a glass composition comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 30 to 60% of La₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂ and B₂O₃ is 14 to 20% and the total content of the above components is at least 95% (to be referred to as “optical glass IV of the present invention” hereinafter),

[0019] (5) a glass preform made of any one of the above glasses I to IV,

[0020] (6) an optical product made of any one of the above glasses I to IV,

[0021] (7) a process for the production of an optical product made of any one of the above optical glasses I to IV, which comprises the steps of melting raw materials for glass and directly press-molding a molten glass, and

[0022] (8) a process for the production of an optical product, which comprises the steps of softening a glass preform made of any one of the above optical glasses I to IV under heat and press-molding the glass preform softened under heat.

PREFERRED EMBODIMENTS OF THE INVENTION

[0023] The optical glass of the present invention includes four embodiments, i.e., four optical glasses I to IV.

[0024] First, the optical glass I is a high-refractivity low-dispersion optical glass having a low glass transition point and having a refractive index nd of at least 1.875, an Abbe's number νd of at least 39.5 and a glass transition point Tg of 700° C. or lower.

[0025] The above optical glass I preferably includes a borosilicate glass containing at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃ and at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 2 to 4, and the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 1 to 2.

[0026] In the composition of the above optical glass I, when the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ [(La₂O₃+Gd₂O₃+Y₂O₃+Yb₂O₃)/(SiO₂+B₂O₃)] is less than 2, it is difficult to obtain an optical glass having a high refractive index and a high Abbe's number, which is one of the objects of the present invention. The content of ZnO that improves dispersion as compared with La₂O₃ and GdO₃ is limited, and as a result, it is difficult to obtain an optical glass having a glass transition point sufficient for mass-productivity. On the other hand, when the above weight ratio exceeds 4, the devitrification resistance is poor, so that it is difficult to obtain a stably mass-producible glass. Therefore, the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is preferably in the range of from 2 to 4, more preferably 3 to 4, still more preferably 3.1 to 3.7.

[0027] Further, when the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ [(ZrO₂+Ta₂O₅+Nb₂O₅)/(SiO₂+B₂O₃)] is less than 1, it is difficult to obtain an optical glass having a high refractive index intended in the present invention and stability sufficient for mass-productivity. On the other hand, when it exceeds 2, the Abbe's number νd decreases, and it is difficult to obtain a low-dispersion optical glass that is an end product of the present invention. Therefore, the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is preferably in the range of from 1 to 2, more preferably 1.1 to 1.5, still more preferably 1.2 to 1.4.

[0028] The above optical glass I may further contain ZnO. The weight ratio of the content of ZnO to the total content of SiO₂ and B₂O₃ [ZnO/(SiO₂+B₂O₃)] is preferably more than 0 but not more than 2, more preferably more than 0 but not more than 1, particularly preferably 0.1 to 0.5. When the [ZnO/(SiO₂+B₂O₃)] weight ratio is in the above range, the glass can be imparted with properties of high refractivity and low dispersion (the dependency of a refractive index on a wavelength is small), and further, the glass can be improved in devitrification resistance and the temperature for the viscous flow thereof can be decreased.

[0029] As a preferred composition, the optical glass I of the present invention has a glass composition (a) comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 0 to 5% of GeO₂, 0 to 15% of ZnO, 30 to 60% of La₂O₃, 0 to 30% of Gd₂O₃, 0 to 10% of Y₂O₃, 0 to 5% of Yb₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂, B₂O₃ and GeO₂ is 14 to 20% by weight, the total content of B₂O₃ and ZnO is at least 9% by weight and the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is 50 to 60% by weight, and further wherein the total content of the above components exceeds 95% by weight, and the composition contains 0 to 1% by weight of Li₂O and 0 to 3% of Nb₂O₅.

[0030] In the above glass composition, SiO₂ is a glass-network-forming component essential for maintaining devitrification resistance, and the content thereof is preferably in the range of from 3 to 10% by weight. When the above content is less than 3% by weight, the devitrification resistance is insufficient. When it exceeds 10% by weight, the refractive index decreases. It is therefore difficult to obtain a high-refractivity optical glass that is an end product of the present invention. In view of the devitrification resistance and the refractive index, the content of SiO₂ is preferably in the range of from 6 to 9% by weight, more preferably 6.5 to 8.5% by weight.

[0031] B₂O₃ is a component having an effect as a network-forming oxide or an effect on meltability of the glass and a decrease in the temperature for viscous flow, and the content thereof is preferably in the range of from 7 to 15% by weight. When the content of B₂O₃ is less than 7% by weight, the effect on meltability of the glass and a decrease in the temperature for viscous flow is not sufficient. When it exceeds 15% by weight, the refractive index of the glass decreases. It is therefore difficult to obtain a high-refractivity glass that is an end product of the present invention. In view of the effect on the meltability of the glass and a decrease in the temperature for viscous flow and the refractive index, the content of B₂O₃ is more preferably in the range of from 9 to 12% by weight, still more preferably 9.5 to 11% by weight.

[0032] GeO₂ has the same effect as that of the above SiO₂, and may be incorporated in an amount in the range of from 0 to 5% by weight. When the content thereof exceeds 5% by weight, the devitrification resistance is liable to decrease.

[0033] The total content of the above SiO₂, B₂O₃ and GeO₂ is preferably in the range of from 14 to 20% by weight. When the above total content is less than 14% by weight, the crystallization tendency increases in intensity, so that it is difficult to obtain an optical glass that can be stably produced. When it exceeds 20% by weight, the refractive index decreases, so that it is difficult to obtain a high-refractivity optical glass that is an end product of the present invention. In view of the crystallization tendency and the refractive index, the above total content of the above SiO₂, B₂O₃ and GeO₂ is more preferably in the range of from 16 to 19% by weight, still more preferably 16 to 18% by weight.

[0034] ZnO works to impart the glass with a high refractive index and low dispersion (the dependency of a refractive index on a wavelength is small), and further, it works to improve the glass in devitrification resistance and to decrease the temperature for the viscous flow. ZnO is therefore a component that is added as required after its amount is adjusted particularly by taking account of the amount of B₂O₃. Specifically, (SiO₂+B₂O₃):ZnO:(La₂O₃+Gd₂O₃+Y₂O₃+Yb₂O₃):(Nb₂O₅+ZrO₂+Ta₂O₅) is adjusted to impart the glass with a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5 and also with devitrification resistance feasible for glass production, and the total content of B₂O₃ and ZnO is adjusted preferably to at least 9% by weight, more preferably to at least 12% by weight, whereby the temperature for viscous flow can be decreased (Tg is 700° C. or lower). The content of ZnO is advantageously in the range of from 0 to 15% by weight. When the content of ZnO exceeds 15% by weight, the devitrification resistance may be insufficient in an attempt to obtain a refractive index in the intended range. On the other hand, when an attempt is made to maintain the devitrification resistance feasible for stable production, the refractive index decreases. It is therefore difficult to obtain a high-refractivity optical glass that is an end product of the present invention. The content of ZnO is more preferably in the range of 1 to 7% by weight, still more preferably 3 to 5% by weight.

[0035] La₂O₃ is an essential component for obtaining a high-refractivity low-dispersion optical glass, and the content thereof is preferably in the range of from 30 to 60% by weight. When the above content is less than 30% by weight, it is difficult to obtain an intended high-refractivity low-dispersion optical glass, and when it exceeds 60% by weight, the devitrification resistance decreases, so that it is difficult to obtain a glass that can be stably produced. The above content is more preferably in the range of from 37 to 48% by weight, still more preferably 40 to 45% by weight.

[0036] Gd₂O₃ may be incorporated in an amount range of from 0 to 30% by weight as a substitute for La₂O₃. When the above content exceeds 30% by weight, the devitrification resistance decreases, so that it is difficult to obtain a glass that can be stably produced. The above content is more preferably in the range of from 0 to 18% by weight, still more preferably 5 to 15% by weight.

[0037] Y₂O₃ and Yb₂O₃ may be also incorporated in the amount range of from 0 to 10% by weight and 0 to 5% by weight, respectively, as a substitute for the above La₂O₃. When the content of Y₂O₃ exceeds 10% by weight, or when the content of Yb₂O₃ exceeds 5% by weight, the devitrification resistance decreases, so that it is difficult to obtain a glass that can be stably produced. The content of Y₂O₃ is more preferably in the range of from 0 to 6% by weight, still more preferably 3 to 6% by weight. Further, the content of Yb₂O₃ is more preferably in the range of from 0 to 5% by weight, still more preferably 0 to 2% by weight.

[0038] The above La₂O₃ and the above Gd₂O₃, Y₂O₃ and Yb₂O₃ have similar effects on optical properties, and preferably, the total content of these components is in the range of from 50 to 60% by weight. When the above total content is less than 50% by weight, it is difficult to obtain a high-refractivity low-dispersion optical glass that is an end product of the present invention. When it exceeds 60% by weight, the devitrification resistance decreases, so that it is difficult to obtain a glass that can be stably produced. The above total content is more preferably in the range of from 51 to 58% by weight, still more preferably 54 to 56% by weight.

[0039] ZrO₂ is a component for attaining a high refractive index, and it exhibits an effect on improving the devitrification resistance when incorporated in a small amount. The content thereof is preferably in the range of from 2 to 8% by weight. When the above content is less than 2% by weight, it is difficult to obtain a high-refractivity optical glass, and ZrO₂ may fail to exhibit the effect on fully improving the devitrification resistance. When the above content exceeds 8% by weight, the devitrification resistance may rather decrease and the glass transition point may increases, so that it may be difficult to achieve the object of the present invention. The above content is more preferably in the range of from 4 to 8% by weight, still more preferably 4 to 6% by weight.

[0040] Ta₂O₅ is an essential component for attaining the high-refractivity, and the content thereof is preferably in the range of from 13 to 19% by weight. When the above content is less than 13% by weight, it is difficult to obtain a high-refractivity optical glass that is an end product of the present invention. When it exceeds 19% by weight, not only the devitrification resistance may decrease, but also the transmittance absorption end may shift toward a longer wavelength side. The above content is more preferably in the range of from 13 to 17% by weight, still more preferably 14 to 17% by weight.

[0041] In the optical glass I of the present invention, preferably, the total content of SiO₂, B₂O₃, GeO₂, ZnO, La₂O₃, Gd₂O₃, Y₂O₃, Yb₂O₃, ZrO₂ and Ta₂O₅ exceeds 95% by weight. When the above total content is less than 95% by weight, it is difficult to obtain an optical glass that satisfies all of the optical properties, the decrease in temperature for viscous flow and the devitrification resistance that are intended in the present invention. The above total content is more preferably at least 96% by weight, still more preferably at least 98% by weight.

[0042] In addition to the above components, the optical glass I of the present invention may contain Nb₂O₃, WO₃, Al₂O₃, Bi₂O₃, Ga₂O₃, BaO, SrO, CaO, MgO, Na₂O, K₂O, Li₂O and Sb₂O₃ as required.

[0043] Nb₂O₃ and WO₃ are components for improving the devitrification resistance when incorporated in a small amount. Each of these components may be incorporated in an amount range of from 0 to 3% by weight. When the content of Nb₂O₃ exceeds 3% by weight, or when the content of WO₃ exceeds 3% by weight, the absorption of the glass in the shorter wavelength region is intensified, which causes the glass to be colored. The content of Nb₂O₃ is more preferably in the range of from 0 to 1.5% by weight, still more preferably 0.5 to 1.5% by weight. Further, the content of WO₃ is more preferably in the range of from 0 to 2% by weight, still more preferably 0 to 1% by weight.

[0044] Bi₂O₃ is a component having an effect on decreasing Tg when incorporated in a small amount, and it may be incorporated in an amount range of from 0 to 3% by weight. When the content of Bi₂O₃ exceeds 3% by weight, it decreases the devitrification resistance or causes the glass to be colored. The content thereof is more preferably in the range of from 0 to 2% by weight, still more preferably 0 to 1% by weight.

[0045] Al₂O₃ and Ga₂O₃ may work to improve the devitrification resistance in some cases when incorporated in a small amount. However, they also work to decrease the refractive index. The content of each of these components is preferably in the range of from 0 to 3% by weight. The content of Al₂O₃ is more preferably in the range of from 0 to 2.5% by weight, still more preferably 0 to 0.5% by weight.

[0046] BaO, SrO, CaO and Mgo have an effect on promoting defoaming when used in the form of carbonates or nitrates as raw materials for the glass. However, when the total content of these components exceeds 3% by weight, the devitrification resistance decreases, so that it is difficult to obtain an optical glass that can be stably produced. The total content of BaO, SrO, CaO and Mgo is therefore preferably in the range of from 0 to 3% by weight. The content of BaO is more preferably in the range of from 0 to 2% by weight, still more preferably 0 to 1% by weight. The content of SrO is more preferably in the range of from 0 to 2% by weight, still more preferably 0 to 1% by weight.

[0047] Na₂O, K₂O and Li₂O have an effect on decreasing the glass transition point Tg, and Li₂O in particular has a very high effect on such. However, these components function as a great factor for decreasing the devitrification resistance and the refractive index. The total content of Na₂O, K₂O and Li₂O is preferably in the range of from 0 to 1% by weight. The content of Li₂O is more preferably in the range of from 0 to 0.5% by weight.

[0048] Sb₂O₃ as a refining agent may be incorporated in an amount range of from 0 to 1% by weight. The above refining agent Sb₂O₃ may be replaced with other refining agent such as SnO₂. The content of Sb₂O₃ is preferably in the range of from 0 to 0.5% by weight.

[0049] As another preferred composition, the optical glass I of the present invention has a glass composition (b) comprising, by % by weight, 5 to 10% of SiO₂, 7 to 13% of B₂O₃, 0 to 5% of GeO₂, 0 to 15% of ZnO, 30 to 60% of La₂O₃, 0 to 30% of Gd₂O₃, 0 to 5% of Y₂O₃, 0 to 5% of Yb₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂, B₂O₃ and GeO₂ is 14 to 20% by weight, the total content of B₂O₃ and ZnO is at least 9% by weight and the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is 50 to 60% by weight, and further wherein the total content of the above components exceeds 95% by weight, the composition further contains, by % by weight, 0 to 3% of Nb₂O₃, 0 to 3% of WO₃, 0 to 3% of Al₂O₃, 0 to 3% of Bi₂O₃, 0 to 3% of Ga₂O₃ and 0 to 1% of Sb₂O₃, the total content of BaO, SrO, K₂O and MgO is 0 to 3% by weight, and the total content of Na₂O, K₂O and Li₂O is 0 to 1% by weight.

[0050] The optical glass II of the present invention is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃ and at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 3.2 to 5, and the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 1.1 to 1.5, the boroslicate glass having a refractive index nd of at least 1.875 and an Abbe's number vd of at least 39.5.

[0051] The weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ [(La₂O₃+Gd₂O₃+Y₂O₃+Yb₂O₃)/(SiO₂+B₂O₃)] is from 3.2 to 5, preferably from 3.2 to 4.5, more preferably from 3.2 to 4, still more preferably from 3.2 to 3.5.

[0052] Further, the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ [(ZrO₂+Ta₂O₅+Nb₂O₅)/(SiO₂+B₂O₃)] is from 1.1 to 1.5, preferably from 1.2 to 1.3.

[0053] While the above optical glass II has a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5, it can exhibit a glass transition point Tg of 700° C. or lower.

[0054] The optical glass III of the present invention is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃ and at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅ and containing ZnO, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 2 to 5, the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 0.5 to 3 and the weight ratio of ZnO to the total content of SiO₂ and B₂O₃ is at least 0.14, the borosilicate glass having a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5.

[0055] In the optical glass III, the weight ratio of ZnO to the total content of SiO₂ and B₂O₃ [ZnO/(SiO₂+B₂O₃)] is at least 0.14, preferably from 0.14 to 2, more preferably 0.18 to 1.5, still more preferably 0.2 to 1. Further, the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ [(La₂O₃+Gd₂O₃+Y₂O₃+Yb₂O₃)/(SiO₂+B₂O₃)] is from 2 to 5, preferably 2.5 to 4.5, more preferably 3 to 3.5.

[0056] Further, the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ [(ZrO₂+Ta₂O₅+Nb₂O₅)/(SiO₂+B₂O₃)] is from 0.5 to 3, preferably from 0.8 to 2, more preferably from 1 to 1.5.

[0057] While the above optical glass III has a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5, it can exhibit a glass transition point Tg of 700° C. or lower.

[0058] As a preferred glass composition, the above optical glasses II and III has a glass composition comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 0 to 5% of GeO₂, 0 to 15% of ZnO, 30 to 60% of La₂O₃, 0 to 30% of Gd₂O₃, 0 to 10% of Y₂O₃, 0 to 5% of Yb₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂, B₂O₃ and GeO₂ is 14 to 20% by weight, the total content of B₂O₃ and ZnO is at least 9% by weight and the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is 50 to 60% by weight, and further wherein the total content of the above components exceeds 95% by weight and the glass composition contains 0 to 1% by weight of Li₂O and 0 to 3% by weight of Nb₂O₅.

[0059] Each component of the above glass composition is as explained with regard to the glass composition (a) in the above optical glass I.

[0060] Further, the optical glass IV of the present invention is an optical glass having a composition comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 30 to 60% of La₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂ and B₂O₃ is 14 to 20% by weight, and the total content of the above components is at least 95% by weight.

[0061] In the above optical glass IV, preferably, part of La₂O₃ is replaced with Gd₂O₃ and/or Y₂O₃, the content of Gd₂o₃ is 0 to 30% by weight, the content of Y₂O₃ is 0 to 10% by weight, and the glass has a glass transition point Tg of 700° C. or lower.

[0062] In the optical glass IV, preferably, ZnO is contained in an amount of 0 to 15% by weight and the total content of ZnO and B₂O₃ is at least 9% by weight. In the optical glass IV, particularly preferably, part of La₂O₃ is replaced with Gd₂O₃ and/or Y₂O₃, the content of Gd₂O₃ is 0 to 30% by weight, the content of Y₂O₃ is 0 to 10% by weight, the content of ZnO is 0 to 15% by weight, the content of Nb₂O₅ is 0 to 3% by weight, the content of Li₂O is 0 to 1% by weight, and the glass has a glass transition point Tg of 700° C. or lower. In the optical glass IV, a reason for limitation of the amount range of each component and a preferred amount range of each component are as explained in the glass composition (a) in the above optical glass I.

[0063] According to the present invention, there is also provided a preform made of any one of the above optical glasses I to IV, and there is also provided an optical product made of any one of the above optical glasses I to IV.

[0064] The above glass preform can be produced by melting raw materials for any one of the above optical glasses I to IV and pre-shaping a molten glass by cold- or hot processing.

[0065] The above optical product can be produced by a process comprising the steps of melting raw materials for any one of the above optical glasses I to IV and directly press-molding a molten glass. Alternatively, the above optical product can be produced by a process comprising the steps of re-heating the above glass preform to a temperature suitable for press-molding (temperature to attain a viscosity of 10⁴ to 10⁷ Pa·s), e.g., approximately 850° C., to soften the preform, and press-molding the preform.

[0066] The above processes may include the step of annealing the press-molded article of the glass after the step of directly press-molding a molten glass or after the step of press-molding the glass preform. The above annealing of the molded article of the glass can be carried out at a temperature around the glass transition point thereof, preferably, in a temperature range within ±20° C. from the glass transition point. In the present invention,,the temperature for the annealing can be set at 720° C. or lower, and it can be set at a 700° C. or lower for some glass compositions.

[0067] In the above manner, optical products such as a lens and a prism can be efficiently produced.

[0068] The optical glass of the present invention is a high-refractivity low-dispersion glass having a low temperature for viscous flow as compared with a conventional optical glass. Therefore, the optical glass of the present invention obviates any annealing or re-heat pressing at a particularly high temperature, so that the optical glass can be stably produced.

[0069] The present invention will be explained more in detail with reference to Examples, while the present invention shall not be limited by any of these Examples.

EXAMPLES 1 - 10 and COMPARATIVE EXAMPLES 1 and 2

[0070] Powdery raw materials of carbonate, nitrate, hydroxide, oxide, and the like were provided for forming a glass composition shown in each Example in Tables 2 to 4, and the raw materials were fully mixed. Then, the mixture was placed in a platinum crucible, melted in a furnace set at 1,400° C., stirred and refined, and the molten glass was cast into a frame made of iron pre-heated to a proper temperature, maintained at a temperature around Tg for 2 hours and then gradually cooled to give an optical glass.

[0071] The thus-obtained optical glasses were measured for properties by the following methods. Tables 2 to 4 also show the results.

[0072] (1) Refractive index [nd] and Abbe's number [νd]

[0073] An optical glass was produced by cooling a molten glass at a temperature-decreasing rate of 30° C./hour and measured.

[0074] (2) Glass transition point Tg

[0075] An optical glass was measured with a thermo-mechanical analyzer at a temperature elevation rate of 4° C./minute.

[0076] (3) Liquidus temperature [L.T.]

[0077] A glass was placed in a platinum crucible having a volume of 50 ml, the crucible with a glass in it was covered and maintained in a furnace at a predetermined temperature for 2 hours and cooled. Then, an inside of the glass was observed through a microscope having a magnification of 100 times, and the liquidus temperature of the glass was determined on the basis of whether or not a crystal was formed. The above temperature was changed at intervals of 10° C.

[0078] (4) λ80

[0079] A polished sample having a thickness of 10 mm was measured for a spectral transmittance, and a wavelength (nm) at a transmittance of 80% was determined. TABLE 2 Example 1 2 3 4 Glass SiO₂ 7.3 7.3 7.3 7.3 Compo- B₂O₃ 8.4 9.4 7.6 9.4 sition GeO₂ 0 0 0 0 (wt %) (SiO₂ + B₂O₃) (15.7) (16.7) (14.9) (16.7) (SiO₂ + B₂O₃ + GeO₂) (15.7) (16.7) (14.9) (16.7) ZnO 2.2 5.0 11.0 5.0 (B₂O₃ + ZnO) (10.6) (14.4) (18.6) (14.4) La₂O₃ 47.8 55.8 41.6 45.8 Gd₂O₃ 10.0 0 10.0 10.0 Y₂O₃ 0 0 0 0 Yb₂O₃ 0 0 0 0 (La₂O₃ + Gd₂O₃ + Y₂O₃ + Yb₂O₃) (57.8) (55.8) (51.6) (55.8) ZrO₂ 5.2 5.2 5.2 7.2 Ta₂O₅ 14.9 15.9 15.9 13.9 (Sub-total) (95.8) (98.6) (98.6) (98.6) Nb₂O₅ 0.8 0.8 0.8 1.2 WO₃ 0 0 0 0 Al₂O₃ 2.6 0.4 0.4 0 Bi₂O₃ 0 0 0 0 BaO 0 0 0 0 SrO 0 0 0 0 Li₂O 0.6 0 0 0 Sb₂O₃ 0.2 0.2 0.2 0.2 ZnO/(SiO₂ + B₂O₃) 0.140 0.299 0.738 0.299 (La₂O₃ + Gd₂O₃ + Y₂O₃ + Yb₂O₃)/ 3.682 3.341 3.463 3.341 (SiO₂ + B₂O₃) (Nb₂O₅ + ZrO₂ + Ta₂O₅)/(SiO₂ + B₂O₃) 1.331 1.311 1.470 1.335 Pro- Liquidus temperature 1290 1280 1290 1260 perties [L.T.] (° C.) Refractive index [nd] 1.88 1.89 1.90 1.89 Abbe's number [vd] 40.8 40.8 39.6 40.7 Glass transition point 692 699 689 697 [Tg] (° C.) λ80 (nm) 471 450 467 450

[0080] TABLE 3 Example 5 6 7 8 Glass SiO₂ 6.7 6.7 7.3 8.3 Compo- B₂O₃ 10.8 10.8 8.4 9.2 sition GeO₂ 0 0 0 0 (wt %) (SiO₂ + B₂O₃) (17.5) (17.5) (15.7) (17.5) (SiO₂ + B₂O₃ + GeO₂) (17.5) (17.5) (15.7) (17.5) ZnO 3.2 4.5 4.5 3.2 (B₂O₃ + ZnO) (14.0) (15.3) (12.9) (12.4) La₂O₃ 41.2 41.8 37.8 47.2 Gd₂O₃ 10.0 9.6 18.0 10.0 Y₂O₃ 6.0 3.8 0 0 Yb₂O₃ 0 0 0 0 (La₂O₃ + Gd₂O₃ + Y₂O₃ + Yb₂O₃) (57.2) (55.2) (55.8) (57.2) ZrO₂ 5.2 5.2 5.2 4.2 Ta₂O₅ 15.9 15.9 13.9 16.9 (Sub-total) (99.0) (98.3) (95.1) (99.0) Nb₂O₅ 0.8 1.3 0 0.8 WO₃ 0 0 1.8 0 Al₂O₃ 0 0 1.9 0 Bi₂O₃ 0 0 1.0 0 BaO 0 0 0 0 SrO 0 0 0 0 Li₂O 0 0.2 0 0 Sb₂O₃ 0.2 0.2 0.2 0.2 ZnO/(SiO₂ + B₂O₃) 0.183 0.257 0.287 0.183 (La₂O₃ + Gd₂O₃ + Y₂O₃ + Yb₂O₃)/ 3.269 3.154 3.554 3.269 (SiO₂ + B₂O₃) (Nb₂O₅ + ZrO₂ + Ta₂O₅)/(SiO₂ + B₂O₃) 1.251 1.280 1.217 1.251 Pro- Liquidus temperature 1260 1250 1290 1270 perties [L.T.] (° C.) Refractive index [nd] 1.88 1.88 1.88 1.88 Abbe's number [vd] 41.2 40.9 40.5 41.0 Glass transition point 699 672 707 713 [Tg] (° C.) λ80 (nm) 443 464 456 450

[0081] TABLE 4 Example CEx. 9 10 1 2 Glass SiO₂ 5.9 6.7 9.3 9.8 Compo- B₂O₃ 12.6 9.7 8.4 14.7 sition GeO₂ 0 1.5 0.5 0 (wt %) (SiO₂ + B₂O₃) (18.5) (16.4) (17.7) (24.5) (SiO₂ + B₂O₃ + GeO₂) (18.5) (17.9) (18.2) (24.5) ZnO 0 3.2 0 4.5 (B₂O₃ + ZnO) (12.6) (12.9) (8.4) (19.2) La₂O₃ 41.2 42.5 43.8 36.8 Gd₂O₃ 11.0 10.0 14.0 7.6 Y₂O₃ 5.0 4.0 0 3.8 Yb₂O₃ 2.0 0 0 0 (La₂O₃ + Gd₂O₃ + Y₂O₃ + Yb₂O₃) (59.2) (56.5) (57.8) (48.2) ZrO₂ 5.4 5.2 5.2 5.2 Ta₂O₅ 15.9 15.9 13.9 15.9 (Sub-total) (99.0) (98.7) (96.1) (98.3) Nb₂O₅ 0.6 0.8 0 1.3 WO₃ 0 0 1.8 0 Al₂O₃ 0 0 1.9 0 Bi₂O₃ 0 0 1.0 0 BaO 0 0.3 0 0 SrO 0 0 0 0 Li₂O 0 0 0 0.2 Sb₂O₃ 0.2 0.2 0.2 0.2 ZnO/(SiO₂ + B₂O₃) 0 0.195 0 0.184 (La₂O₃ + Gd₂O₃ + Y₂O₃ + Yb₂O₃)/ 3.200 3.445 3.266 1.967 (SiO₂ + B₂O₃) (Nb₂O₅ + ZrO₂ + Ta₂O₅)/(SiO₂ + B₂O₃) 1.184 1.335 1.079 0.914 Pro- Liquidus temperature 1280 1270 1290 1240 perties [L.T.] (° C.) Refractive index [nd] 1.88 1.88 1.88 1.86 Abbe's number [vd] 41.2 41.1 40.7 42.3 Glass transition point 708 707 735 690 [Tg] (° C.) λ80 (nm) 444 460 460 440

[0082] As shown in Tables 2 to 4, the glasses of the present invention have a refractive index nd of at least 1.875 and an Abbe's number ν of at least 39.5, and it is seen that the glasses in Examples 1 to 6 have a glass transition point Tg of 700° C. or lower and that the glasses in Examples 7 to 10 have a glass transition point Tg of 707 to 713° C.

[0083] In the glass in Comparative Example 1, the weight ratio of (Nb₂O₅+ZrO₂+Ta₂O₅)/(SiO₂+B₂O₃) is 1.079 or less than 1.2, and the glass has a high glass transition point Tg of 735° C. In the glass in Comparative Example 2, the weight ratio of (La₂O₃+Gd₂O₃+Y₂O₃+Yb₂O₅)/(SiO₂+B₂O₃) is 1.967 or less than 3.1, and the glass has a low refractive index of 1.86.

[0084] When the optical glass obtained in Example 6 of the present invention was maintained in an electric furnace at 850° C. for 5 minutes, it was sufficiently softened. The optical glass obtained in Comparative Example 1 was hardly softened. This difference shows that the optical glass of the present invention can be re-heat pressed at a lower temperature than a conventional optical glass.

EXAMPLE 11

[0085] Raw materials were melted in a silica crucible or a platinum crucible for forming the glass composition in each of Examples 1 to 10, and glasses were formed. Glass blocks were taken from these glasses and cut to obtain molding glass materials.

[0086] Each molding glass material was individually softened under heat until they had a viscosity of 10⁴ to 10⁷ Pa·s, introduced into a mold having a molding surface corresponding to an optically functional surface of an optical product as an end product, and press-molded to give glass molded materials. Then, the glass molded materials were respectively annealed at glass transition points Tg thereof, to produce optical products.

EXAMPLE 12

[0087] Raw materials were melted in a silica crucible or a platinum crucible for forming the glass composition in each of Examples 1 to 10. Each molten glass was respectively adjusted to have a viscosity of at least 0.3 Pa·s and flowed down from a feeder, and a predetermined amount of each glass that had flowed down was cast into a mold and press-molded to give glass molded materials. Then, the glass molded materials were respectively annealed at glass transition points Tg thereof, to produce optical products.

[0088] Effect of the Invention

[0089] The optical glass of the present invention is a high-refractivity low-dispersion glass having a low temperature for viscous flow, so that it serves to attain stable production of optical products without annealing or re-heat pressing at particularly high temperatures. 

What is claimed is:
 1. An optical glass having a refractive index nd of at least 1.875, an Abbe's number νd of at least 39.5 and a glass transition point Tg of 700° C. or lower.
 2. The optical glass of claim 1, which is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃ and at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 2 to 4 and the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 1 to
 2. 3. The optical glass of claim 2, which further contains ZnO whose weight ratio to the total content of SiO₂ and B₂O₃ is more than 0 but not more than
 2. 4. The optical glass of claim 3, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 2 to 4, the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 1 to 2 and the weight ratio of ZnO to the total content of SiO₂ and B₂O₃ is from 0.1 to 0.5.
 5. The optical glass of claim 1, which has a glass composition comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 0 to 5% of GeO₂, 0 to 15% of ZnO, 30 to 60% of La₂O₃, 0 to 30% of Gd₂O₃, 0 to 10% of Y₂O₃, 0 to 5% of Yb₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂, B₂O₃ and GeO₂ is 14 to 20% by weight, the total content of B₂O₃ and ZnO is at least 9% by weight, the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is 50 to 60% by weight and the total content of the above components is at least 95% and further wherein the composition contains, by % by weight, 0 to 1% of Li₂O and 0 to 3% of Nb₂O₅.
 6. The optical glass of claim 5, which contains, by % by weight, 9 to 12% of B₂O₃ and 1 to 7% of ZnO and has a B₂O₃ and ZnO total content of at least 12% by weight.
 7. The optical glass of claim 5 or 6, which contains, by % by weight, 6 to 9% of SiO₂, 9 to 12% of B₂O₃ and 0 to 5% of GeO₂ and has an SiO₂, B₂O₃ and GeO₂ total content of 16 to 19% by weight.
 8. The optical glass of claim 1, which has a glass composition comprising, by % by weight, 5 to 10% of SiO₂, 7 to 13% of B₂O₃, 0 to 5% of GeO₂, 0 to 15% of ZnO, 30 to 60% of La₂O₃, 0 to 30% of Gd₂O₃, 0 to 5% of Y₂O₃, 0 to 5% of Yb₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂, B₂O₃ and GeO₂ is 14 to 20% by weight, the total content of B₂O₃ and ZnO is at least 9% by weight and the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is 50 to 60% by weight, and further wherein the total content of the above components exceeds 95% by weight, the composition further contains, by % by weight, 0 to 3% of Nb₂O₃, 0 to 3% of WO₃, 0 to 3% of Al₂O₃, 0 to 3% of Bi₂O₃, 0 to 3% of Ga₂O₃ and 0 to 1% of Sb₂O₃, the total content of BaO, SrO, K₂O and MgO is 0 to 3% by weight, and the total content of Na₂O, K₂O and Li₂O is 0 to 1% by weight.
 9. An optical glass which is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃ and at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 3.2 to 5 and the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 1.1 to 1.5, and which has a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5.
 10. An optical glass which is a borosilicate glass comprising at least one selected from La₂O₃, Gd₂O₃, Y₂O₃ or Yb₂O₃, at least one selected from ZrO₂, Ta₂O₅ or Nb₂O₅ and ZnO, wherein the weight ratio of the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ to the total content of SiO₂ and B₂O₃ is from 2 to 5, the weight ratio of the total content of ZrO₂, Ta₂O₅ and Nb₂O₅ to the total content of SiO₂ and B₂O₃ is from 0.5 to 3 and the weight ratio of ZnO to the total content of SiO₂ and B₂O₃ is at least 0.14, and which has a refractive index nd of at least 1.875 and an Abbe's number νd of at least 39.5.
 11. The optical glass of claim 9 or 10, which has a glass composition comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 0 to 5% of GeO₂, 0 to 15% of ZnO, 30 to 60% of La₂O₃, 0 to 30% of Gd₂O₃, 0 to 10% of Y₂O₃, 0 to 5% of Yb₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂, B₂O₃ and GeO₂ is 14 to 20% by weight, the total content of B₂O₃ and ZnO is at least 9% by weight and the total content of La₂O₃, Gd₂O₃, Y₂O₃ and Yb₂O₃ is 50 to 60% by weight, and further wherein the total content of the above components exceeds 95% by weight and the glass composition contains 0 to 1% by weight of Li₂O and 0 to 3% by weight of Nb₂O.
 12. The optical glass of claim 11, which contains, by % by weight, 9 to 12% of B₂O₃ and 1 to 7% of ZnO and has a total content of B₂O₃ and ZnO of at least 12% by weight.
 13. The optical glass of claim 11 or 12, which contains, by % by weight, 6 to 9% of SiO₂, 9 to 12% of B₂O₃ and 0 to 5% of GeO₂ and has an SiO₂, B₂O₃ and GeO₂ total content of 16 to 19% by weight.
 14. An optical glass comprising, by % by weight, 3 to 10% of SiO₂, 7 to 15% of B₂O₃, 30 to 60% of La₂O₃, 2 to 8% of ZrO₂ and 13 to 19% of Ta₂O₅, wherein the total content of SiO₂ and B₂O₃ is 14 to 20% by weight, and the total content of the above components is at least 95% by weight.
 15. The optical glass of claim 14, wherein part of La₂O₃ is replaced with Gd₂O₃ and/or Y₂O₃, the content of Gd₂O₃ is 0 to 30% by weight, the content of Y₂O₃ is 0 to 10% by weight, the optical glass containing 0 to 15% by weight of ZnO, and further wherein the total content of ZnO and B₂O₃ is at least 9% by weight, the optical glass having a glass transition point Tg of 700° C. or lower.
 16. The optical glass of claim 14, wherein part of La₂O₃ is replaced with Gd₂O₃ and/or Y₂O₃, the content of Gd₂O₃ is 0 to 30% by weight, the content of Y₂O₃ is 0 to 10% by weight, the content of ZnO is 0 to 15% by weight, the content of Nb₂O₅ is 0 to 3% by weight and the content of Li₂O is 0 to 1% by weight, the optical glass having a glass transition point Tg of 700° C. or lower.
 17. A glass preform made of the optical glass recited in claim 1, 9, 10 or
 14. 18. An optical product made of the optical glass recited in claim 1, 9, 10 or
 14. 19. A process for the production of the optical product recited in claim 18, which comprises the steps of melting raw materials for a glass and directly press-molding a molten glass.
 20. The process of claim 19, which further comprises the step of annealing a glass molded material obtained by the press-molding, after the step of directly press-molding a molten glass.
 21. A process for the production of an optical product, which comprises the steps of softening the glass preform recited in claim 17 under heat and press-molding the glass preform softened under heat.
 22. The process of claim 21, which further comprises the step of annealing a glass molded material obtained by the press-molding, after the step of press-molding the glass preform. 